1. Field of the Invention
This invention pertains generally to devices and methods for the production of x-rays, and more particularly to a pyroelectric or piezoelectric crystal based generator of x-rays that is light weight, compact and does not need a large external power supply. Field emitters formed as micrometer-scale exposed regions in the crystal having one or more sharp peaks or ridges emit electrons that impact a bremsstrahlung target to produce x-rays. A matrix or mosaic of crystals may also be used in place of a single crystal.
2. Description of Related Art
A wide variety of medical diagnostic imaging and treatment systems, industrial testing systems and security scanning systems are centered on the differences in the absorption of x-rays by different materials. The resolution of the two dimensional image of a three dimensional object produced by conventional imaging devices is often dependent on several parameters including time of exposure to x-rays and the intensity of the beam.
The conventional x-ray tube, in either rotating anode or Crooke's-tube-like configurations, is the workhorse of many medical imaging systems. While there have been countless refinements, the basic mechanism of x-ray imaging with the x-ray tube has remained unchanged for decades. Conventional x-ray tubes typically consist of an evacuated housing containing an anode and a cathode. Cathodes are electron emitting filaments often made of tungsten, aluminum, titanium or steel. Cold cathodes sometimes include a rare earth coating to enhance electron emission. Anodes are often made of metals such as molybdenum, palladium, tungsten, copper and silver.
A high-voltage supply is utilized to create an arc or discharge of electrons between the negative electron emitting cathode and the positive anode. Within the arc are electrons with kinetic energies at or near the applied potential that are accelerated through the electric field between the cathode and anode. When these accelerated electrons strike a target (typically the anode), x-rays are produced through Bremsstrahlung (“braking radiation”) as well as other ionization processes (e.g. inner shell electron “characteristic radiation”).
Conventional tubes can be relatively light (a few kilograms) and fragile, since they are fabricated from glass. However, the power supplies are typically large, expensive and heavy (in the tens of kilograms). The majority of the applied power goes into waste heat, requiring cooling and further adding to the bulk and weight of conventional x-ray devices.
A number of technologies have been considered to reduce the size and weight of x-ray sources. X-ray microtubes are an attempt to construct millimeter-scale devices by miniaturizing the conventional tube design. These devices still require external high voltage supplies (as well as water cooling, in some cases); however, the weight of the unit itself (excluding the power supply and cooling system) is very low. As they are typically designed for cancer therapy, where dose precision is critical, the designs are not optimized for cost. A typical number of treatment cycles per tube is ˜10.
Field emitters of electrons have been investigated in a number of contexts by a variety of researchers. In principle, it is known that such field emitters or arrays of these emitters are able to produce x-rays by irradiating a bremsstrahlung target with electrons. The energy of the electrons, and hence of the x-rays emitted, is directly proportional to the applied voltage. Maintaining a sufficiently high voltage (30-120 kV) across a tiny gap without breakdown is very challenging and has been a barrier to miniaturization. (Field emitter arrays used in e.g. plasma televisions operate at only a few hundred volts.) A variant technology, the cold-cathode emitter array, has also been developed, and flat-panel x-ray sources based on this technology are being brought to market. This approach seems very promising, but still requires a significant external power supply.
Radioactive sources can also provide a good source of x-rays. Co-60-based-sources are still in use in developing countries for medical and dental x-rays. However, concerns about safety and nuclear material proliferation make these systems very undesirable. Moreover, shielding of the sources implies that devices tend to be very heavy on the order of hundreds of kilograms.
There is one known x-ray source based on the pyroelectric effect. It relies on a bulk pyroelectric crystal emitting electrons that then impact a copper target. The resultant x-rays are emitted through a beryllium window of 15 mm diameter. This source is able to produce sporadic fluxes of x-rays over a small emission area, and does not require a high-voltage power supply. However, it lacks repeatability or control and also has a nearly random output flux.
Accordingly, there is a need for a flat panel source of x-rays which is robust and portable and does not need a large power supply. X-ray sources can follow a similar development path televisions and video displays that have moved from tube-based technologies to flat screens. The present invention satisfies this need, as well as others, by providing an addressable modular array of x-ray sources that is self contained and useable in remote locations.